An analysis of the kinds of behavior included under the concept of "spontaneous activity" suggests that it might profitably be partitioned into 1) non-specific restless activity, sensitive to perturbations of the internal milieu and the level of external stimulation; 2) specific responses such as grooming, rearing, scratching, etc. which have social, ecological or physiological significance; and 3) responses involving substantive expenditures of energy which are involved in regulation of body composition and are reinforced by their effects on metabolic surrogates of body composition. The implications of this hypothesis are developed and research proposed to test them, thus clarifying the nature of a potentially useful reinforcer. The relations between this research and the problems of diet, obesity, disease stress, hyperactivity, and the effect of activity on learning and performance are explored.